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DETACHED-EDDY SIMULATIONS OF ATMOSPHERIC FLOW OVER …
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The 13th OpenFOAM Workshop (OFW13), June 24-29, 2018, Shanghai, China DETACHED-EDDY SIMULATIONS OF ATMOSPHERIC FLOW OVER COMPLEX TERRAINS GUOLEI WANG 1 , PANKAJ JHA 1 , GREGORY OXLEY 1 1 Envision Energy, [email protected] 1 Envision Energy, [email protected] 1 Envision Energy,[email protected] Keywords: Wind farm simulation, hybrid RANS/LES, atmospheric boundary layer Introduction As the growing requirement from renewable energy, especially for wind energy, more and more on-shore wind farms has been constructed on the mountain ridges and plateaus. In contrast to the off-shore or flat terrain wind farms, reliably modelling the atmospheric flow over the complex terrains is a big challenge. A separation bubble, induced by the rolling hills includes low wind speed and high turbulence intensity. If the wind turbine happened to be located in this region, a low annual energy production (AEP) and fatigue failure of the wind turbine will be caused. Negative wind shear, which means the wind speed in the lower part of the turbine is higher than that in the higher part, will cause the turbine blade collide with the turbine tower, resulting the broken of the blade. Such unfavourable wind conditions will cause a finance loss, resulting a big risk of the project. Currently, Reynolds-Averaged Navier Stokes (RANS) with two-equation turbulence model (k-ε model) is a standard process for the wind resource assessment and the siting optimization. By considering the measured met mast data and a standard post-processing form IEC, the wind resource file can be obtained and will be used to represent a 20 years mean value for the siting engineers. A following siting optimization and AEP calculation can be done to evaluate the risk of the project. However, it has been reported that steady RANS failed to accurately predict the flow features when flow separation occurs [1] . Large-Eddy Simulation, which only model the sub-grid scale turbulence, is a promising method to increase the accuracy when solving the wind farm. But due to the consuming of the calculation, it usually used to simulate the turbine-scale flows, such as SOWFA [2] . Detached-Eddy Simulation (DES), which will use RANS method in the near wall region and switches to LES method in the turbine region, can provide us more information than the traditional RANS method. It can be used to investigate the unsteady features in the wind farm, such as the possibility of the negative turbine wind shear and wind veer [3] . Compared with RANS method, DES method shows better performance when solving the hill induced separation [4] . Greenwich CFD (GWCFD), is a wind resource assessment CFD software developed by Envision Energy based on OpenFOAM. It integrates the pre-processing of the wind farm engineering, such as map and mast data clean, mesh generation and the post-processing, like cross-checking and the annual energy production (AEP). The primary workhorse of Greenwich CFD is the neutral RANS solver with k-ε closure, which carries 90% of the internal workload from the operational siting engineers in Envision Energy. In addition, Greenwich CFD has developed advanced models, such as thermal stratification model of atmosphere boundary layer, forest model, actuator line/disk model, as well as meso/micro scale coupling model. Description of the case In this paper, a wind farm in China is selected to investigate as shown in Figure 1. Wind turbines with hub height of 44m are located along the hill. The length of the plateau is about 1000m from southeast to northwest and about 10km from southwest to northeast. The hill elevation ranges from 1100m to 1700m. A met mast is located on the northeast of the wind farm. The wind rose at the measured met mast is shown in Figure 2, with the main wind direction of about 135degree (The direction of the wind from north is defined as 0 degree). The hill slope is about 30deg in the windward side. Based on the result from RANS, the predicted wind speed (or AEP) on the upwind side of the turbine group is higher than that on the downwind side. While the operating data from Year 2012 to Year 2015 shows the AEP on the upwind side is 30% lower than predicted. A detached-eddy simulation method with the standard K-Omega SST model [5] is used to review this case. Parameters in the turbulence model are carefully selected to make them consistent with the RANS k-Epsilon model [6] . Wind conditions at the problematic turbines will be fully considered. Results and Discussion 30
Transcript of DETACHED-EDDY SIMULATIONS OF ATMOSPHERIC FLOW OVER …
The 13th OpenFOAM Workshop (OFW13), June 24-29, 2018, Shanghai, China
DETACHED-EDDY SIMULATIONS OF ATMOSPHERIC FLOW OVER COMPLEX
TERRAINS
GUOLEI WANG1, PANKAJ JHA1, GREGORY OXLEY1 1Envision Energy, [email protected]
1Envision Energy, [email protected] 1Envision Energy,[email protected]
Keywords: Wind farm simulation, hybrid RANS/LES, atmospheric boundary layer
Introduction
As the growing requirement from renewable energy, especially for wind energy, more and more on-shore wind farms
has been constructed on the mountain ridges and plateaus. In contrast to the off-shore or flat terrain wind farms, reliably
modelling the atmospheric flow over the complex terrains is a big challenge. A separation bubble, induced by the
rolling hills includes low wind speed and high turbulence intensity. If the wind turbine happened to be located in this
region, a low annual energy production (AEP) and fatigue failure of the wind turbine will be caused. Negative wind
shear, which means the wind speed in the lower part of the turbine is higher than that in the higher part, will cause the
turbine blade collide with the turbine tower, resulting the broken of the blade. Such unfavourable wind conditions will
cause a finance loss, resulting a big risk of the project.
Currently, Reynolds-Averaged Navier Stokes (RANS) with two-equation turbulence model (k-ε model) is a standard
process for the wind resource assessment and the siting optimization. By considering the measured met mast data and a
standard post-processing form IEC, the wind resource file can be obtained and will be used to represent a 20 years mean
value for the siting engineers. A following siting optimization and AEP calculation can be done to evaluate the risk of
the project. However, it has been reported that steady RANS failed to accurately predict the flow features when flow
separation occurs [1]. Large-Eddy Simulation, which only model the sub-grid scale turbulence, is a promising method to
increase the accuracy when solving the wind farm. But due to the consuming of the calculation, it usually used to
simulate the turbine-scale flows, such as SOWFA [2]. Detached-Eddy Simulation (DES), which will use RANS method
in the near wall region and switches to LES method in the turbine region, can provide us more information than the
traditional RANS method. It can be used to investigate the unsteady features in the wind farm, such as the possibility of
the negative turbine wind shear and wind veer [3]. Compared with RANS method, DES method shows better
performance when solving the hill induced separation [4].
Greenwich CFD (GWCFD), is a wind resource assessment CFD software developed by Envision Energy based on
OpenFOAM. It integrates the pre-processing of the wind farm engineering, such as map and mast data clean, mesh
generation and the post-processing, like cross-checking and the annual energy production (AEP). The primary
workhorse of Greenwich CFD is the neutral RANS solver with k-ε closure, which carries 90% of the internal workload
from the operational siting engineers in Envision Energy. In addition, Greenwich CFD has developed advanced models,
such as thermal stratification model of atmosphere boundary layer, forest model, actuator line/disk model, as well as
meso/micro scale coupling model.
Description of the case
In this paper, a wind farm in China is selected to investigate as shown in Figure 1. Wind turbines with hub height of
44m are located along the hill. The length of the plateau is about 1000m from southeast to northwest and about 10km
from southwest to northeast. The hill elevation ranges from 1100m to 1700m. A met mast is located on the northeast of
the wind farm. The wind rose at the measured met mast is shown in Figure 2, with the main wind direction of about
135degree (The direction of the wind from north is defined as 0 degree). The hill slope is about 30deg in the windward
side. Based on the result from RANS, the predicted wind speed (or AEP) on the upwind side of the turbine group is
higher than that on the downwind side. While the operating data from Year 2012 to Year 2015 shows the AEP on the
upwind side is 30% lower than predicted. A detached-eddy simulation method with the standard K-Omega SST model [5] is used to review this case. Parameters in the turbulence model are carefully selected to make them consistent with the
RANS k-Epsilon model [6]. Wind conditions at the problematic turbines will be fully considered.
Results and Discussion
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The 13th OpenFOAM Workshop (OFW13), June 24-29, 2018, Shanghai, China
The computational domain is 20km*20km, with the horizontal mesh resolution of 12.5m and the first wall normal mesh
of 1m, resulting a total mesh size of about 70million. A log law inflow profile is used to simulate the neutral atmosphere
boundary layer[1,3]. A rough wall function is used to model the near wall roughness effect [7]. Flow features in the
dominant wind direction (135 degree) is simulated and analysed in this paper.
Figure 3 shows the flow pattern along the main wind direction near the problematic wind turbine. A separation bubble
is observed in the leeside of the hill. The flow is disturbed on the plateau. The zoomed-in result at four time steps are
shown in Figure 4. The turbine suffers unsteady wind conditions during the operation. The history of the wind speed
variation at the upwind and downwind turbines is recorded during the simulation. Two probes at turbine bottom
(H=20m) and turbine top (H=60m) are used to check the wind speed difference on the turbine disk as shown in Figure 5.
The wind shear defined as (dU/dz), is analysed based on the probe data. It is found that the possibility of the negative
wind shear is 65% for the upwind turbine and only 19% for the downwind turbine. Since large negative wind shear can
cause the turbine blade collide with turbine tower, It is a high risk for the wind turbine in the upwind side.
Figure 1. Terrain information of the site
Figure 2. Wind rose of the Met mast
Figure 3. Flow pattern along the problematic wind turbine plane.
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The 13th OpenFOAM Workshop (OFW13), June 24-29, 2018, Shanghai, China
Figure 4. Zoomed-in flow patterns at different time near the problematic turbines.
Figure 4. Time history of the wind speed variation at the upwind and downwind turbines.
References
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flow models[J]. Boundary-Layer Meteorology, 2011, 141(2): 245
[2] Churchfield M, Lee S, Moriarty P, et al. A large-eddy simulation of wind-plant aerodynamics[C]//50th AIAA
Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition. 2012: 537
[3] Hu C H. Investigation of unfavorable winds associated with complex terrain using Detached Eddy
Simulation[J]
[4] Bechmann A, Sørensen N N. Hybrid RANS/LES method for wind flow over complex terrain[J]. Wind Energy,
2010, 13(1): 36-50
[5] Menter F R, Kuntz M, Langtry R. Ten years of industrial experience with the SST turbulence model[J].
Turbulence, heat and mass transfer, 2003, 4(1): 625-632
[6] Sogachev A, Kelly M, Leclerc M Y. Consistent two-equation closure modelling for atmospheric research:
buoyancy and vegetation implementations[J]. Boundary-layer meteorology, 2012, 145(2): 307-327
[7] Richards P J, Hoxey R P. Appropriate boundary conditions for computational wind engineering models using
the k-ε turbulence model[M]//Computational Wind Engineering 1. 1993: 145-153
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